EFFECT OF DEXAMETHASONE ADMINISTRATION ON SERUM THYROID-HORMONE CONCENTRATIONS IN CLINICALLY NORMAL HORSES

The effect that 5 consecutive days of treatment with dexamethasone (0.04 mg/kg of body weight, IM, q 24 h) would have on baseline concentrations of triiodothyronine (T-3), thyroxine (T-4), reverse T-3 (rT(3)), free T-3 (FT3), and free T-4 (FT4), and on response to thyroid-stimulating hormone (TSH) administration was determined in 12 clinically normal horses. Results of TSH response tests indicated that the horses could be placed into 2 groups: in 6 horses (group A), T-4 concentration after administration of TSH was less than twice the baseline concentration. Baseline serum concentrations of T-3, T-4, rT(3), FT3, and FT4 were not significantly different between group-A adn group-B horses. In both groups of horses, serum T-3, T-4, rT(3), and FT4 concentrations were significantly increased 6 hours following TSH administration, compared with baseline concentrations. Treatment with dexamethasone resulted in significant (P < 0.05) increases in baseline concentrations of rT(3) and FT3 in group-A horses and baseline concentrations of rT(3) in group-B horses. The response to TSH administration following dexamethasone treatment appeared to be blunted with significant (P < 0.05) increases only in T-3, T-4, and FT4 concentrations in group-A horses and FT4 concentration in group-B horses. The magnitude of change in serum FT3 concentration in response to TSH administration was significantly less (P = 0.05) following dexamethasone treatment, compared with magnitude of change prior to dexamethasone treatment. Results suggested that TSH response testing may not be as valuable as once thought for diagnosing hypothyroidism in horse or for differentiating thyroidal from nonthyroidal illness.